1. Technical Field
The present invention relates to an electrophoretic display device and an electronic apparatus.
2. Related Art
As electrophoretic display devices of an active matrix type, devices having a switching transistor and a memory circuit (SRAM: static random access memory) within each pixel have been known (see US 2008/0238867). Furthermore, the applicant of the invention has proposed an electrophoretic display device in which a switching circuit is disposed within a pixel in addition to the memory circuit (see US 2008/0238867).
FIG. 16A is a circuit diagram of a pixel 540 of an electrophoretic display device 500 having the above-described configuration disclosed in US 2008/0238867. FIG. 16B is a schematic cross-section view of a display unit 505 of the electrophoretic display device 500.
As shown in FIG. 16A, the pixel 540 includes a selection transistor 41, a latch circuit 70, a switching circuit 580, a pixel electrode 35, an electrophoretic element 32, and a common electrode 37. In addition, to the pixel 540, a scanning line 66, a data line 68, a high-electric potential power supplying line 50, a low-electric potential power supplying line, a first control line 91, and a second control line 92 are connected.
As shown in FIG. 16B, in the display unit 505 of the electrophoretic display device 500, a plurality of pixel electrodes 35A and 35B are disposed. In addition, between the pixel electrodes 35A and 35B and the common electrode 37 facing both the pixel electrodes 35A and 35B, an electrophoretic element 32 having microcapsules 20 is pinched. The microcapsules 20 and the pixel electrodes 35A and 35B are bonded though an adhesive agent layer 33.
In addition, circuit elements, electrodes, the electrophoretic element, and the like that are shown in FIG. 16 will be described later in detail with reference to FIG. 2 and the like.
The electrophoretic display device 500 disclosed in US 2008/0238867 employs a configuration in which the switching circuits 580 are controlled in accordance with a voltage maintained in the latch circuit 70 and an electric potential (S1 or S2) is input to the pixel electrode 35 by connecting the pixel electrode 35 to any between two control lines 91 and 92 through the switching circuit 580. According to such an electrophoretic display device 500, there are advantages that display of an intermediate gray scale or partial rewriting of the display unit can be performed by controlling the electric potentials of the first and second control lines 91 and 92, and a leakage current between pixels can be decreased.
However, in order to implement new high-level functions and low power consumption of the electrophoretic display device, there are the following problems in the electrophoretic display device disclosed in US 2008/0238867.
In the electrophoretic display device 500, a boundary between a pixel of white display and a pixel of a black display can be displayed clearly. However, when a straight line or a curve that extends in the inclining direction with respect to the arrangement direction of the pixels is represented, there is a problem that jaggies (saw teeth shapes) are recognized visually. For this point, in the electrophoretic display device 500, to be described later in detail, partial rewriting can be performed for the display unit. Accordingly, by disposing a display area of an intermediate gray scale in a boundary portion between white display and black display, an anti-aliasing process can be performed. However, according to the above-described driving method, since image data for displaying the intermediate gray scale needs to be transmitted to the pixel, there are problems that consumption of a current for driving a driver increases, and a time required for completing display is lengthened.
In addition, there is a problem that power consumption increases due to inter-pixel leakage currents in an electrophoretic display device of a microcapsule type, which is not limited to the electrophoretic display device 500. In particular, as shown in FIG. 16B, when a pixel 540A of the black display and the pixel 540B of the white display are located to be adjacent to each other, an electric field E is formed between a pixel electrode 35A having a high-level electric potential VH (for example, 15 V) and a pixel electrode 35B having a low-level electric potential VL (for example, 0 V) in the horizontal direction (substrate surface direction). Thus, inter-pixel leakage currents are generated by the electric field E under the influence of little moisture contained in the adhesive agent layer 33 that bonds the microcapsules and the pixel electrode. In addition, there is a problem that power consumption increases due to the inter-pixel leakage currents.
In addition, generation of the leakage currents due to influence of little moisture and the like represents a possibility that an electrochemical reaction may occur between the pixel electrode 35 and the adhesive agent layer 33. In other words, ionic migration and corrosion that decrease the reliability of the pixel electrodes 35 may occur. When precious metal such as gold or platinum is used as a formation material of the pixel electrode, the reliability is improved. However, by using the precious metal, the cost increases, and the manufacturing process becomes complicated. As a result, it is difficult to suppress the manufacturing cost while improving the reliability.
In the electrophoretic display device 500, when partial rewriting driving is to be performed, the first control line 91 or the second control line 92 that is connected to the pixel electrode 35 of the pixel 540 in which display is not to be changed is in the high impedance state.
FIGS. 17A, 17B, and 18 are explanatory diagrams showing a partial rewriting driving process. FIG. 17A is an explanatory diagram showing a planar structure of the display unit 505 of the electrophoretic display device 500. FIG. 17B is an explanatory diagram showing the cross-section structure of the pixels 540A to 540D shown in FIG. 17A. In addition, FIG. 18 is an explanatory diagram showing the circuit configuration of the pixels 540A, 540E, and 540F shown in FIG. 17A.
In addition, constituent elements shown in FIGS. 17A, 17B, and 18 are described in detail later. The subscripts of “A” to “F” assigned in reference signs are only for identifying a plurality of the pixels 540 and constituent elements thereof.
In the electrophoretic display device 500, when only one pixel 540A is to be rewritten, as shown in FIGS. 17A and 17B, the pixel electrode 35A of the pixel 540A to be rewritten and the first control line 91 are electrically connected through the switching circuit 580A, and the pixel electrodes 35B to 35F of pixels 540B to 540D in which display is maintained and the second control line 92 are electrically connected through switching circuits 580B to 580F. Then, the high-level electric potential VH (for example 15 V) is supplied to the first control line 91, the second control line 92 is set to be in the high impedance state, and the low-level electric potential VL (for example, 0 V) is input to the common electrode 37.
In such a case, in the pixel 540A, the electrophoretic element 32 is driven in accordance with an electric potential difference between the pixel electrode 35A having the high-level electric potential VH and the common electrode 37 having the low-level electric potential VL, and whereby black display is represented. On the other hand, in the other pixels 540B to 540F, the pixel electrodes 35B to 35F are in the high impedance state, and accordingly, there is no electric potential difference between the pixel electrodes 35B to 35F and the common electrode 37, and whereby display is maintained.
In the above-described partial rewriting driving process, display of the pixels 540B to 540F of which pixel electrodes 35B to 35F are in the high impedance state is not changed. However, there is a problem that the contrast actually decreases.
As described above, in the electrophoretic display device of a microcapsule type, inter-pixel leakage is generated though the adhesive agent layer 33. Accordingly, as shown in FIGS. 17A, 17B, and 18, even in the partial rewriting driving process, inter-pixel leakage currents Lk are generated between the pixel electrode 35A of the pixel 540A to be rewritten and the pixel electrodes 35B and 35E that are adjacent to the pixel electrode 35A. Accordingly, an electric potential due to the leakage is input to the pixel electrodes 35B and 35E of the pixels 540B and 540E in which display is maintained.
In such a case, the pixel electrodes 35B to 35F of the pixels 540B to 540E are electrically connected through the second control line 92, and thus, the electric potentials of the pixel electrodes 35B and 35E are supplied to other pixel electrodes 35C and 35F and the like that are adjacent thereto. Then, in the state in which the electric potential is input to the pixel electrodes 35B to 35F as described above, when an image displaying operation is performed by inputting, for example, the low-level electric potential VL to the common electrode 37, display of the pixels 540B to 540F changes, and whereby the whole contrast of the display unit 505 decreases.